It has recently been disclosed by the research group of the present inventors that a styrenic polymer having a syndiotactic configuration is obtained by polymerizing a styrenic monomer by the use of a catalyst comprising as primary ingredients a transition-metal compound, especially a titanium compound and an alkylaluminoxane (Refer to Japanese Patent Application Laid-Open Nos. 187708/1987, 179906/1988, 241009/1988, etc.).
Methylaluminoxane, particularly suitable as an alkylaluminoxane, is obtained usually by the reaction between trimethylaluminum and water. However, the above-mentioned reaction involves the problem that the reaction is difficult to put into practical production because of its violent reaction, and further suffers the disadvantages that in addition to requiring expensive trimethylaluminum as the starting material, an excessively large amount of methylaluminoxane is required as a component of the catalyst as compared with the amount of a transition-metal, thus causing an extremely expensive catalyst, and further that after the polymerization reaction, an additional step for the removal of the catalyst remaining in the resultant polymer is required.
There has recently been reported that a polymerization catalyst not containing aluminoxane is capable of polymerizing .alpha.-olefin(principally, ethylene) by R. Taube (J. Organomet. Chem. C9-C11, 347 (1988)), H. Turner (J. Am. Chem. Soc. 111,2728 (1989)), R. F. Jordan (Organomet. 8,2892 (1989)), etc. Nevertheless, investigation has not yet been made on a polymerization catalyst not containing aluminoxane for the polymerization of a styrenic monomer, leaving the problem that a styrenic monomer, different from an .alpha.-olefin, is likely to be polymerized into an atactic polymer in the presence of a cationic species.
Under such circumstances, intensive research and investigation were concentrated by the present inventors into the development of a catalyst capable of efficiently polymerizing a styrenic monomer into the styrenic polymer having syndiotactic configuration without the use of expensive aluminoxane required in a large amount and the development of the process for producing the styrenic polymer by the use of the aforementioned catalyst.